The invention relates to a method for determining the phosphorus content of an aqueous sample, as well as to an apparatus with which to implement the method.
A known way to determine the amount of particular substances contained in water—and hence the quality of the water and in particular of drainage water contaminated with organic materials, nitrogen compounds and/or halogen compounds—is to vaporize a sample in an atmosphere comprising a transport gas (carrier gas) enriched with oxygen and burn it, and to deliver the gas mixture obtained by this combustion to a detector suitable for demonstrating the presence of carbon dioxide, nitrogen oxides and the like.
As detectors the following (among others) have proved useful: infrared detectors for the carbon content, special chemoluminescence detectors or electrochemical sensors for the nitrogen content, and so-called coulometric detectors for the halogenide content.
The detection methods based on the combustion of a water sample have been widely used to determine the sample's content of organic substances, the so-called TOC (total organic carbon). For this purpose a small amount of the water with the transport gas is customarily placed in an oven heated to a preset temperature by resistance heating; here it is almost immediately vaporized and burnt, and the resulting gas is sent to an NDIR-CO2 detector, which signals a CO2 content that indicates the C content of the water sample. A more advanced embodiment of this procedure and a corresponding apparatus are described in DE 43 44 441 C2. A modified arrangement for measuring very low TOC values—for instance in extremely clean water or very pure solutions for medical applications is described in EP 0 684 471 A2.
Further developed methods of this kind and appropriately designed reactors or complete arrangements have been proposed by the applicant in EP 0 887 643 B1 and EP 1 055 927 B1.
In addition to the above-mentioned materials contained in water, phosphorus is also a chemical element which can substantially affect the quality of drainage water and the techniques needed to process such water, depending on the amount present; for some time, therefore, increased attention has been directed to its quantitative measurement. Unlike the water contents carbon, nitrogen and halogenides, phosphorus has not thus far been quantitatively demonstrable in a gaseous medium (combustion gas+carrier gas). Instead, detection methods that have proved useful for this purpose are implemented with an aqueous solution. These methods employ, to put it simply, the change in colour of an aqueous sample to which a special reagent has been added; they are known as “blue method” or “yellow method” and are specified in corresponding standards.
Whereas phosphorus occurs in natural water in three fractions, namely as (1) inorganic, dissolved orthophosphate, (2) dissolved organic phosphorus compounds and (3) particulate phosphorus bound to biomasses or attached to particles, the known demonstration methods are based on measurement of the orthophosphate content. In order to determine the entire phosphorus content of an aqueous sample, therefore, an assessment of fractions (2) and (3) is needed, by converting them to photometrically detectable orthophosphate.
For this conversion a method has long been known that employs oxidation with addition of chemicals in an acidic medium, in some cases with increased pressure and an elevated reaction temperature; cf. for instance JP 2004093509 A. A known means of determining the phosphorus content in an organic material (e.g., oil) is to mix this material with an alkaline solution and burn it under oxygen atmosphere in a hermetically sealed reaction vessel; cf. JP 62003643 A.
The U.S. Pat. No. 5,702,954 describes a several-step procedure for analysis of plant or animal phosphate-containing samples, which includes combustion in the presence of a reduction means (e.g., hydrogen) followed by conversion with ozone in another reaction chamber at ambient temperature. US 2003/0032194 A1 also describes a several-step oxidation method which was developed primarily for detecting nitrogen and sulphur, but also phosphorus, in a sample containing these elements. Thermal decomposition methods employing special catalyzers or ozone are also known, for example, from JP 59154358 A or JP 61140836 A.
Various other known methods make use of photo-oxidative decomposition of the sample under LTV radiation, in particular in the presence of a photo-oxidation catalyzer. Methods of this kind are described, e.g., in EP 0 634 646 B1 or JP 07027706 A.